An evacuated double-tubing type solar heat collector, which is known in the art, includes an outer tube and an inner tube having a so-called selective absorption film disposed on its outer surface. The inner tube is inserted within the outer tube and the space between the tubes is hermetically sealed and evacuated to a reduced pressure. Since it is difficult to retain the inner tube within the outer tube in a stable manner, the conventional solar heat collector is prone to strain and breakage if subjected to the slightest impact.
One example of the solar heat collector of the aforementioned type has a coil spring for applying an elastic force in the direction of thrust. The coil spring is disposed in the space between the tubes at the base end of the collector. However, it is difficult to maintain coincidence between the central axes of the inner and outer tubes at all times, more specifically, it is difficult to keep the inner tube centered in the outer tube. Even the slightest deflection or off-centering of the inner tube at its base end will give rise to strain within the glass which will in turn cause breakage of the portion where the opening between the tubes is fused closed to provide the hermetic seal.
In another example of a conventional collector tube, the outer tube is provided at a point near its base end with a dimple projecting radially inwardly toward the center of the tube for the purpose of retaining the inner tube by contacting its outer surface at a point near the base end of the tube. The major difficulty with this arrangement is that a gap can form between the dimple and the inner tube. Even a very small gap will permit the inner tube to deflect and strike the dimple, giving rise to a rattling noise. If the gap is large the inner tube will be free to deflect to a greater extent and will result in the type of breakage mentioned above.
In view of the foregoing problems, some solar heat collectors have a gently tapered volute spring fit over the outer periphery of the inner tube. Although such a spring makes it possible to center the inner tube, deflection of the inner tube cannot be prevented because of the elasticity of the spring. Another disadvantage is that the spring slides along the central axis of the inner tube when the inner tube is inserted. Thus, it is inevitable that a projection, such as the above-mentioned dimple, is used in combination with this arrangement.
The present invention has been devised in view of the foregoing shortcomings encountered in the conventional inner tube retaining devices and has as its object to provide an evacuated double-tubing type solar heat exchanger that completely eliminates these shortcomings.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.